Geochemical evaluation of resource potential in mafic-ultramafic intrusions: A case study on Ni and Co in the Hongshishan complex of Gansu Province, China

Abstract

Mafic-ultramafic intrusions are closely associated with NiCo mineralization. Geochemical evaluation of resource potential in mafic-ultramafic intrusions plays a crucial role in guiding NiCo mineral exploration. The NiCo prospecting potential in the Hongshishan mafic-ultramafic complex in Gansu Province is under consideration. A detailed geochemical exploration of the stream sediment-soil-rock system in the Hongshishan area was conducted in this study to illustrate the method for geochemical evaluation of Ni and Co resource potential in mafic-ultramafic intrusions. The stream sediments originated from the regional geochemical survey with a scale of 1:200,000, the soils were from the detailed geochemical survey with a scale of 1:50,000, and the rocks were collected from the Hongshishan complex and its surrounding strata. For elemental concentrations, a nineteen levels' fixed-value method of concentrations (19LsFC) is used to classify Ni and Co concentrations. In the Hongshishan complex, Ni and Co concentration levels are consistently above the 10th level, even reaching the cut-off grade at the 19th level. Furthermore, their concentrations increase systematically from stream sediments through soils to rocks. For elemental anomalies, a seven levels' classification method of anomalies (7LsAn) is adopted to classify geochemical anomalies of Ni and Co. The anomaly levels of Ni and Co are all higher than the 3rd level and even up to the 7th level (or the cut-off grade level), and increase from stream sediments to rocks in the Hongshishan complex region. Here the 19LsFC on Co is presented on eighteen fixed values, including the analytical detection limit, the median value of surface soils in China, the cut-off grade, etc. The 19LsFC is valid to compare elemental concentrations between different elements and samples in a uniform “ruler”. The 7LsAn is constructed on two “nails” (the background value and the cut-off grade value) for a single sample and is also useful for comparing different elements and samples. The illustration in the Hongshishan area provides a new geochemical approach for NiCo resource exploration in mafic-ultramafic intrusions.